1. Field of the Invention
This invention relates generally to semiconductor processing and, more specifically to a buffered slurry for use in chemical mechanical polishing of metals, such as titanium or aluminum. In particular, this invention relates to a slurry which has a pH within the passivation range of titanium or aluminum.
2. Description of the Related Art
Metal layers are used for a variety of purposes in the fabrication of integrated circuits. For example, it is well known that metal layers may be used to form interconnective lines, contacts and other conductive features on and above the surface of a semiconductor wafer. Titanium and aluminum are widely used to form such metal layers. During the fabrication process, removal of a metal layer or portion of a metal layer may be required in order to pattern and form various features. Traditionally, this removal has been accomplished predominantly by wet or dry etching techniques well known in the art.
Recently, there has been a great deal of interest in another technique for removing layers known as chemical mechanical polishing (CMP). CMP is a process in which a polishing pad and slurry are used to remove layers from the upper surface of an in-process semiconductor wafer. Mechanical movement of the pad relative to the semiconductor wafer provides an abrasive force for removing the exposed surface layer of the wafer. Because of the broad surface area covered by a pad in most instances, CMP is often used to planarize a given layer across an entire wafer.
A CMP slurry serves multiple roles; namely, it is the medium in which the abrasive particles is dispersed, and secondly it furnishes the chemical agents which promote the chemical process. In order for optimum results in CMP processing, there must be a synergistic relationship between the chemical and mechanical processes. For example, in prior art CMP slurries for polishing a metal layer, a metal etchant, metal oxidizer, and an abrasive agent have been employed. The oxidant reacts with the metal to form a passive oxide layer, which serves to protect the metal from the etchant. During the polishing process, the abrasive agent removes the passive oxide layer from elevated portions of the metal layer, allowing the metal etchant to etch away a portion of the metal layer. Such etching may not be desirable, though, as dishing of the metal layer may result. Once the metal has been etched, the passive oxide layer forms again. Depressed portions of the metal layer surface are not subject to mechanical abrasion, and therefore are not etched away. This process continues until the elevated portions of the metal layer have been etched away, resulting in planarization. To achieve proper planarization, it is desirable that the slurry not etch or corrode the metal in the absence of the abrasive action provided during the CMP process.
CMP processes for polishing aluminum and aluminum based metals have used acidic slurries having pH ranges of less than about 4 or have used highly basic slurries having pH values of approximately 10 or above. For example, CMP slurries available from "CABOT" and "SOLUTION TECHNOLOGY" are employed for polishing with a pH value of about 2.2, and a CMP slurry available from "RODEL" is employed for polishing with a pH value of about 3.76. A CMP slurry available from "FUJIMI" is employed for polishing with a basic pH value of about 10. Although these very acidic and basic prior art CMP slurries may achieve up to two times the removal rate of a more neutral prior art slurry, overall results are less than optimal. In particular, the very acidic slurry solutions result in the occurrence of a dissolution type of chemistry during the CMP process. The disadvantage of dissolution chemistry is that it causes "dishing" to occur. "Dishing" is a term used to describe the creation of a depressed area in a portion of a metal layer surface during the polishing process. Dishing interferes with the creation of a smooth planarized surface. In addition, the acidic slurry solutions of the prior art tend to corrode polishing tool components, such as the conditioning end-effector and any non-plastic parts of the slurry removal system. Furthermore, some prior art CMP slurry solutions for polishing aluminum suffer from stability problems.
Thus, there is need for a suitable CMP slurry for the planarization of aluminum and other metals that achieves high removal rates in the absence of dissolution chemistry and dishing, and without damage to polishing tool components.